The invention relates to a device and method for reducing the nonsteady side loads acting on a nozzle of a rocket engine, particularly during the startup or ignition phase of said engine.
The invention can be used in particular to eliminate, or at least substantially limit, the nonsteady side loads occurring in the nozzle of a rocket engine during the ignition thereof, caused by the effect known as jet separation, or the separation of the boundary layer, or a separation by internal recirculation in the jet.
It is known that the thrust of a rocket engine depends on its mass flow rate, on the stagnation pressure ps in the combustion chamber, on the nozzle expansion ratio, that is, on the ratio ps/pe between the stagnation pressure ps and the static ejection pressure of the gases at the nozzle exit Pe, and on the ambient pressure pa. This thrust reaches a peak, for a given set of chamber operating conditions, when the two pressure values Pe and pa are equal (the nozzle is then said to be adapted). It is also known that rocket engines are normally designed to reach the matching condition Pe=pa at an altitude higher than the launch altitude, for example, of about 10 000 m and that, in consequence, at low altitude, Pe<pa (operating condition of the nozzle in overexpansion). If the static ejection pressure of the gases at the nozzle exit pe is significantly lower than the ambient pressure pa (for example, lower than 0.2 pa), a jet separation occurs inside the divergent portion of the nozzle. Two types of separation are known: separation by free shock wave (also called free shock separation in English publications) and separation by internal recirculation in the jet (also called restricted shock separation in English publications). For certain nozzle geometries and/or under certain expansion ratio conditions, the boundary layer of the supersonic gas flow separates from the wall of the divergent portion of the nozzle, in a separation mode called a free shock wave separation mode, and the jet is compressed by an oblique shock wave and a side load is applied locally at every point of the wall of said divergent portion downstream of the separation point. This side load is created by the pressure difference between the outer wall of the divergent portion to which the atmospheric pressure is applied and the inner wall of the divergent portion to which the local static pressure of the jet is applied. If the jet separation were perfectly symmetrical and stable over the whole circumference of the nozzle and at a defined axial position, the local static pressure of the jet would be uniform over the circumference of the nozzle, and the resultant of these side loads would be zero. In reality, the jet separation line has an irregular and highly nonsteady shape. It follows that at every instant, the jet separation produces a nonzero resultant load, which may have a considerable moment with respect to the throat of the nozzle, at the place where the structural moment of inertia of the engine is the lowest. This clearly explains why the most critical situation occurs when the jet separation mainly takes place on a single side of the nozzle and close to the exit section. For other nozzle geometries, or with other expansion ratio conditions for the same nozzle, a different separation mode may occur, called separation mode by internal recirculation in the jet, and which also generates harmful nonsteady side loads. In this separation mode, as in the previous one, the boundary layer of the supersonic gas flow separates from the wall of the divergent portion of the nozzle, but, because of the level of the downstream pressures, the flow immediately reattaches to the wall of the divergent portion, thereby forming a toroidal separation bulb. The position of this toroidal separation bulb is controlled by a shock created at the center of the flow by a broad gas recirculation that shifts randomly, randomly affecting the position of the central shock and the position of the toroidal separation bulb. Downstream of the toroidal separation bulb, the jet remains supersonic and attached to the wall, but it is compressed by an oblique shock wave of which the intensity varies with the Mach number of the incident flow and therefore with the position of the toroidal separation bulb. As a result, the static pressures at the wall downstream of the toroidal separation bulb vary randomly. As in the case of the free shock wave separation, a side load is locally applied at every point of the wall of said divergent portion downstream of the separation point. This side load is created by the pressure difference between the outer wall of the divergent portion to which the atmospheric pressure is applied and the inner wall of the divergent portion to which the local static pressure of the jet is applied. If the toroidal separation bulb were perfectly symmetrical, coaxial with the nozzle, stable over time throughout the circumference of the nozzle and at a defined axial position, the recompression due to the shock that it generates would be uniform and the local static pressure of the jet downstream would be uniform over the circumference of the nozzle, and the resultant of these side loads would be zero. In reality, for the reasons set forth above, the separation line of the toroidal separation bulb has an irregular and highly nonsteady shape. It follows that at every instant, the separation by internal jet recirculation induces a nonzero resultant load, which may have a considerable moment with respect to the throat of the nozzle, at the place where the structural moment of inertia of the engine is the lowest. This clearly explains why the most critical situation appears when the separation by internal jet recirculation mainly takes place in a half along a vertical cross section of the nozzle and close to its exit section.
The need to maintain the nonsteady loads induced by the jet separation at an acceptable level requires limiting the value of the expansion ratio ps/pe to below its optimal value and overdimensioning the nozzle structure, thereby reducing the overall performance of the engine and its thrust/weight ratio. Despite these precautions, the nonsteady loads generated by the jet separation cause considerable vibrations that are liable to damage the nozzle and even to cause it to break if, with the passage of time, the random pressure distribution in the divergent portion becomes excessively unfavorable.
A thorough analysis of the jet separation mechanisms in rocket engine nozzles and the resulting nonsteady side loads is provided in the article by G. Hagemann, M. Terhardt, M. Frey, P. Reijasse, M. Onofri, F. Nasuti and J. Östlund, “Flow Separation and Side-Loads in Rocket Nozzles”, presented to the 4th International Symposium on Liquid Space Propulsion, Lampoldshausen, Germany, 13-15 March 2000.
Numerous devices have been proposed for controlling the jet separation inside a rocket engine nozzle in order to limit said nonsteady side loads, and in particular the following.
Document U.S. 6,572,030 discloses the use of a droppable annular structure, extending radially and designed to be placed around the nozzle exit section. This structure causes the formation of a low pressure zone close to said exit section, thereby reducing the jet separation inside the nozzle.
Document U.S. 5,894,723 discloses the use of ejectable inserts inside the nozzle. Following ascension, the ejection of said inserts increases the ratio of the area of the nozzle exit section to the area of its throat, thereby enabling the engine to operate in near matching conditions throughout the rocket ascension phase.
Document U.S. 5,490,629 discloses the use of an ejectable diffuser, connected to the nozzle exit section and having a contraction to recompress the gases and thereby prevent the jet separation during the first part of the rocket's trajectory.
Document U.S. 5,481,870 discloses the use of a droppable annular obstacle, connected to the nozzle exit section and partially obstructing it so as to cause a stable jet separation.
Document U.S. 5,450,720 discloses the use of longitudinal slots in the downstream end portion of the nozzle to cause a stable jet separation.
All these documents disclose solutions to the problem of eliminating or limiting the nonsteady side loads generated in the nozzle of a rocket engine during the first part of its ascent phase from liftoff to the altitude at which the matching condition is reached. However, none of the devices described therein is suitable for limiting the appearance of nonsteady side loads while the stagnation pressure in the engine combustion chamber has not yet reached its nominal value, that is, even before the liftoff of the rocket, during the engine startup phase. During this phase, which lasts about one second or slightly less, the stagnation pressure ps of the gases in the combustion chamber increases rapidly from atmospheric to a peak value and, in consequence, the mean position of the jet separation line shifts toward the nozzle exit section, making ineffective the known control means of the prior art, of which the geometric definition is fixed with respect to the nozzle. Furthermore, these documents propose the use of devices which are integral with the nozzle during at least part of the ascent phase of the rocket, and which thereby increase its weight, which is contrary to one of the goals of controlling the jet separation, which is to lighten the nozzle by reducing the loads to which it is subjected.
The only known prior art document that presents a solution to the problem of limiting the nonsteady side loads during the engine startup phase without making the nozzle heavier, is document FR 2 791 398, which discloses a system for stabilizing the jet separation, comprising a device outside the engine, integral with a ground installation, consisting of a set of injection tubes sending countercurrent fluid jets into the nozzle toward impact points on the wall thereof. A jet separation region is produced from each impact point and extends toward the nozzle exit section in a conical configuration. Such a system causes an overall reduction of the nonsteady side loads in the nozzle and has the advantage of being mounted on a ground installation and not carried by the rocket itself, but it is not fully satisfactory because it does not effectively stabilize the jet separation throughout the engine startup phase, since the countercurrent jets impact the nozzle at positions that are fixed and independent of the pressure in the combustion chamber. Furthermore, as shown in FIG. 1 of document FR 2 791 398, the jet separation lines originating from each impact point intersect the edge of the nozzle exit section. In fact, these lines have a stable position at the impact point of the countercurrent fluid jet that initiates them, but can fluctuate downstream of this point, thereby inducing residual nonsteady loads. This is particularly undesirable because these residual loads are applied correspondingly from the edge of the nozzle exit section, that is, at the place where they are most detrimental, because their moment with respect to the nozzle throat is at maximum.